ZOOKEEPER-3630: Autodetection of openssl during ZooKeeper C client build

**Thanks for ztzg for raising the issue and suggesting the solution!**

In this patch we enhance the way how the openssl library is found during C client build. I introduced  and documented a new build parameter for `configure` (`--with-openssl=...`), `cmake` (`-D WITH_OPENSSL=...`) and `mvn` (`-Dc-client-openssl=...`), so independent of the build tool, the build will work the same way:
- By default, the OpenSSL library will be autodetected. If the library is found, then the C-client will be compiled with SSL support, otherwise we get a warning message, but the build will continue without SSL support. The SSL related unit tests will be skipped as well.
- you can explicitly disable the SSL support (e.g. `-Dc-client-openssl=no`)
- or you can specify an alternative path to look for the openssl library (e.g. `-Dc-client-openssl=/path/to/openssl`)

I tested the patch
- using mvn on linux
- using make on linux
- using cmake on linux and on windows

In addition, I also added the openssl dependencies to the dev docker image and copied the OpenSSL license to the C client LICENSE file.

Author: Mate Szalay-Beko <szalay.beko.mate@gmail.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, Norbert Kalmar <nkalmar@cloudera.com>, Damien Diederen

Closes #1159 from symat/ZOOKEEPER-3630

1 files changed, 9 insertions, 2 deletions

diff --git a/README_packaging.md b/README_packaging.md
index e2a2d4767..b290dd9f9 100644
--- a/README_packaging.md
+++ b/README_packaging.md
@@ -57,7 +57,13 @@ Optional parameters you might consider when using maven:
 -  `-Pfull-build`         -   activates the full-build profile, causing the C client to be built
 -  `-DskipTests`          -   this parameter will skip both java and C++ unit test execution during the build
 -  `-Pc-test-coverage`    -   activates the test coverage calculation during the execution of C client tests
-
+-  `-Dc-client-openssl`   -   specify ssl support and openssl library location. Default value: `yes`, resulting in 
+                              the autodetection of the openssl library. If the openssl library will not be detected, 
+                              then a warning will be shown and the C client will be compiled without SSL support.
+                              Use `-Dc-client-openssl=no` to explicitly disable SSL feature in C client. Or use 
+                              `-Dc-client-openssl=/path/to/openssl/` if you want to use a non-default / specific 
+                              openssl library location.
+    
 Please note: if you don't provide the `-Pfull-build` parameter, then the C client will not be built, the C client tests
 will not be executed and the previous C client builds will no be cleaned up (e.g. with simply using `mvn clean`).
 
@@ -66,4 +72,5 @@ The compiled C client can be found here:
 - `zookeeper-client/zookeeper-client-c/target/c/lib`                 - Native libraries
 - `zookeeper-client/zookeeper-client-c/target/c/include/zookeeper`   - Native library headers
 
-The same folders gets archived to the `zookeeper-assembly/target/apache-zookeeper-<version>-lib.tar.gz` file, assuming you activated the `full-build` maven profile.
+The same folders gets archived to the `zookeeper-assembly/target/apache-zookeeper-<version>-lib.tar.gz` file, assuming 
+you activated the `full-build` maven profile.